1. Field of the Invention
This invention generally relates to the field of semiconductor processing and more particularly to improving end point detection by preventing the accretion of by-products on end point windows.
2. Description of the Relevant Art
Fabrication of an integrated circuit entails the sequencing of numerous processing operations. During the manufacture of an integrated circuit various layers of dielectric, polysilicon, and metal are deposited, doped, patterned, etched, and polished to form specific features of the circuit such as gates, interconnects, and contacts. For some of these processes, determining the end point of the processing cycle plays an important role in achieving high throughput and run-to-run reproducibility. Ideally, the end point of a processing cycle could be theoretically predicted as a function of the parameters that effect the processing environment. In this way, a semiconductor processing tool could be calibrated to terminate a processing step at a predetermined time corresponding to the end point of the processing cycle. However, because it is difficult to control all of the process parameters that affect the end point of a cycle, precise calibration is not always feasible.
To provide a more reliable means for end point detection, reaction chambers are typically designed with a window or optical port for use in conjunction with an external end point detector. End point windows are utilized in a wide variety of semiconductor processing tools including etch tools. Generally an end point detector includes an optical sensor positioned outside the window to receive light from inside the reaction chamber. Changes in the characteristics of the light signal that the end point of the processing cycle has been reached. However, chemical reactions ongoing during processing can lead to the buildup of opaque material such as polymer on interior surfaces of the reaction chamber including the end point window. Over time polymer accumulates on the end point window. Eventually, after a number of processing runs the window becomes so dirty that the optical sensor cannot accurately detect when the processing cycle ends (i.e. the end point).
One process for which end point detection is critical is dry etching, also known as plasma etching. The plasma etch process removes a patterned material from the surface of an underlying thin film using gases as the primary etch medium and plasma energy to drive the reaction. As integrated circuit technology pushes further into the sub-micron device realm, dry etching has become a mainstay process in semiconductor fabrication. This is because dry etching is generally anisotropic, allowing the controlled formation of devices with small feature dimensions. Careful monitoring of the end point of a dry etch cycle is essential to achieving high yield goals. Otherwise, overetch in a directional (anisotropic) pattern can damage underlying features.
There are basically three methods of optical end point detection: (1) optical emission spectrometry; (2) laser interferometry and reflectance; and (3) direct observation of a process through a viewing port by a human operator. See, e.g., Wolf, Silicon Processing in the VLSI Era, pp. 565-567 for a general discussion of these methods of end point measurement. The most common method for determining the end point of a dry etch is optical emission spectrometry. Optical emission spectroscopy is well known in the etching arts as a method for determining the end point of dielectric, polysilicon, and metal etches. Reactants and products in the plasma emit light at characteristic wavelengths in transitioning from excited states. The emission intensity at a given wavelength depends on the relative concentration of the species in the plasma that is emitting the light. As the etch progresses, changes in etch chemistry can be observed by monitoring changes in emission intensity of the plasma components. Thus, in the absence of the material to be etched the reactant concentration will be at some equilibrium value. However, as the desired material is being etched, the concentration of reactant species will be at a lower level than it would in the absence of the etched material. When the end point is reached and the etch material has been consumed, the reactant concentration should increase back to its equilibrium value. By calibrating the optical sensor in the presence and absence of the etch material to the signature spectrum of the reactant, the end point can be determined. In a like manner, product species of the etch process can be used to determine the end point of the etch. Optical emission spectroscopy provides a highly sensitive means for determining end points, which presents a further advantage in being easy to implement.
Another method of optical end point detection uses laser interferometry and laser reflectance. An end point detector utilizing laser interferometry and reflectance focuses a laser on a flat region of a film being etched and measures the intensity of light reflected by the film. Whether interferometry or reflectance is used depends on the properties of the layer being etched. Laser interferometry is appropriate when a transparent film such as SiO.sub.2 is being etched, and laser reflectance is utilized when a nontransparent film is being etched. A number of drawbacks are associated with laser interferometry and reflectance. For example, these techniques may not be useful if a large batch of wafer is being etched because these techniques require that a laser be trained on a specific area of a single wafer. Etching information provided by these methods is limited to that confined area of the single wafer on which the laser is focused. Thus, in the case where a large batch of wafers is being processed, laser interferometry and reflectance cannot compensate for non-uniformities in the batch etching process.
The final method of optical end point detection relies on the human eye as the optical sensor. A human operator monitoring the etch process observes the wafer surface being etched through a viewing port on the etch chamber. Direct observation by a human observer is the least reliable of the popular end point detection methods.
While end point detection methods that rely on optical sensing present individual advantages and problems, all suffer from a common drawback attributed with polymer accretion on the end point window. When the window becomes so dirty that end point detection fails, processing must be terminated. In order to restore adequate end point detection capabilities, the etch tool must be opened, cleaned, and requalified to run. A similar procedure is followed for other semiconductor processes that use an optical port as part of an end point detection scheme.
Cleaning and requalifying the etch tool can waste over twelve hours of production time. To avoid costly shutdowns and improve throughput and run-to-run reproducibility, a number of methods have been employed in the past to reduce the problem of polymer accretion on end point windows. One method found in the prior art relies on mechanically wiping or scraping away polymer that has accumulated on the end point window. For example, an etch tool may be modified with a windshield wiper type of device. Such a device essentially consists of an axle passing through the wall of the etch tool near the end point window. The end of the axle extending outside the etch tool is fitted with a knob, and the other end of the axle protruding into the reaction chamber is fitted with a scraper. By turning the knob located on the outside of the tool the scraper rotates and removes accumulated polymer. The principle problem associated with using this type of polymer removal method is contamination of the processing environment. Particulates of polymer that have been scraped off of the window may gravitate towards and accumulate on the wafers being processed providing an undesirable contamination source.
Another method for dealing with polymer accretion on end point windows operates by covering the inside of the window until an end point measurement is to be made. One type of apparatus that could be used in this method is a shutter. During an etch process, the shutter would be in a closed position covering the end point window. In this way, polymer accumulates on the shutter instead of the window. Periodically the shutter would be opened to make an end point measurement. While this method avoids the particulate contamination problem of the scraper method, it suffers from other problems. First, a precise calculation of the end point time is not feasible for reasons already outlined. Therefore, the exact point at which the shutter should be opened cannot be accurately predicted. For this reason, attempting to time the opening of the shutter with the end point is likely to result in over etching. To deal with end point uncertainty and to avoid over etching the shutter will have to be remain open for a long interval around the average end point time, or the shutter will have to be opened frequently during this interval. However, when the shutter is open polymer will still buildup on the window. As the number of processing runs increases, the cumulative open shutter time will lead to some point where end point detection fails because of polymer accretion.
Yet another method for solving the polymer accretion problem uses heat to clean or prevent polymer buildup on end point windows. Polymer will not form on a window that is heated to a sufficient temperature. Moreover, once polymer does form on an end point window heat can drive away the polymer. Of course, burning off the polymer once it has formed on the window can lead to particulate contamination of the wafers being processed. Thus, prevention is preferred over cleaning. Consequently, heating the end point window can avoid the problems accompanying the use of scrapers or shudders. To date, end point windows have been heated via resistive heating of the metal surrounding the window. For example, a wire could be coiled around the window in contact with the metal surrounding the window. A high voltage source would then produce the heating current in the wire. Heat conducts through the metal to the window. End point windows can be made of glass or quartz. However, because glass and quartz are poor thermal conductors, uneven heating of the window will result with less heat being conducted to the inner portion of the window. For this reason, current heating methods for preventing polymer accretion on end point windows are useful for small windows. For larger windows, polymer will still tend to form in the center of the window. This can cause problems for end point detectors where optical sensors must be trained on a specific area of the wafer in the reaction chamber.
Another problem associated with resistive heating involves the placement of the high voltage source near the window. The high voltage may interfere with the end point detector, so that inaccurate measurements are made on the end point of the processing cycle. It would be advantageous to prevent the accretion/accumulation of opaque material such as polymer on an end point window by uniformly heating the window without relying on high voltage resistive heating.
Other optical ports may be incorporated into a semiconductor processing tool for viewing aspects of the process other than end point detection. Therefore, it would be advantageous to have a general method and apparatus for preventing unwanted accretion of opaque material on any optical port, not merely those used for end point detection.